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Exercise in the heat: The “off-switch,” the limiting temperature, and the Beijing Olympics

We’re back with more on fatigue, and as promised, the strategy we’ll adopt over the next few posts is to look at fatigue in very specific situations. These situations, often the intervention controlled by the scientists, offer a glimpse into how exercise performance is either limited or regulated, and help us understand how performance might be improved (by working backwards from the regulation or limit).

As we discussed in our last post, studies can look at exercise performance as being “limited” by some failure, or as a “regulated” process, where the body aims to maintain homeostasis by regulating what we’ve defined as the pacing strategy.

In today’s post, we look at exercise in the heat. This is especially topical this year, because the Beijing Olympics promises to bring the influence of heat and humidity into the public eye in a big way. Elite athletes the world over are preparing for the heat by using special chambers to replicate the likely Beijing conditions, and it’s also one of the reasons we discussed a few weeks ago that the Kenyan runners, the big favourites for the marathon, might have their work cut out, given the “levelling effect” that the heat can have.

Exercise in the heat: What ultimately limits performance?

We start our investigation of the heat by asking this question, which represents, of course, the “limitations” model for exercise. This is a crucial question, however, because if we want to know how exercise is regulated, it’s important to recognize that it is ultimately limited by some variable.

The early theory – blood supply limitations

And until about 30 years ago, the early understanding of exercise in the heat is that it was limited because the body did not have enough blood to get to both the muscles and the skin, where it was needed for cooling. The result of this limited blood supply was that the muscles were deprived of oxygen, became anaerobic, and exercise stopped. Alternatively, the blood pressure was challenged to the point where exercise was completely impossible. This would, according to our discussion of constant workload vs. self-paced exercise, represent the point at which the “bridge breaks”, or the light goes off!

However, in 1979, a scientist called Nadel published a study showing that blood flow was in fact not limiting during exercise in hot conditions. This was followed by studies in the 1990’s from Denmark (where a lot of heat research comes from) which showed the same thing – there may be a challenge to blood supply during exercise in the heat, but the body is more than capable of meeting it in healthy individuals. And so that theory was disproven.

A clue to the limit – mental confusion

But around the same time, it was recognized that when these athletes were exercising in the laboratory, there came a point at which they actually developed mental symptoms – lack of co-ordination, dizziness, confusion and loss of ability to control their limbs! This led scientists to speculate that in fact, the limit to exercise in the heat was central, involving the brain. The speculation at the time, as far back as 1987 by a Canadian pair (Bruck and Olschiewski), was that a high body temperature affected brain function and the drive to exercise.

The famous video, shown at the end of this post, captures this situation – it is Gabrielle Andersen, staggering and swaying through the Olympic stadium in Los Angeles, typifying the human response observed in the research subjects at the point of exhaustion during exercise in the heat – paralysis on one side of the body, confusion, loss of co-ordination and balance.

Limiting body temperature – the “off-switch”

Subsequent work showed this “central fatigue hypothesis” to be a distinct possibility. It turned out that animals and humans all stopped exercise at a very distinct body temperature.

For example, in cheetahs (running on treadmills, believe it or not!), it was noticed as far back as 1973, that at a particular point, the animals displayed very strange behaviour – they simply “gave up” running and lay down! In the words of the authors (Taylor and Rowntree):

…the cheetahs refused to run… They would simply turn over with their feet in the air and slide on the tread(mill) surface.

Later, it became possible to actually measure the body temperature of animals and humans during exercise (I can’t imagine it’s very easy to measure the body temperature of a Cheetah during running! Rats and goats, perhaps, are easier propositions!). It was found that all animals seemed to have a very narrow range of body temperatures at which they would stop exercise. For example, beagles stopped at body temperatures of about 42 degrees, antelope 42 degrees, and goats 43 degrees. Rats, the most tested of all, were found to stop at about 41 degrees celsius.

Human beings – a thermal limit to exercise and a proposed mechanism

Then came humans. And perhaps not surprisingly, research found that humans tended to stop at a body temperature of about 40 degrees celsius. What was most interesting is that this temperature was consistent regardless of pre-cooling, the rate of heat storage, and the degree of heat adaptation. In other words, it seemed that humans have this “off-switch” at 40 degrees celsius, irrespective of the external intervention. The only thing that changed was the time it took to get there – for example, a person who is well adapted to the heat is able to sweat more, lose more heat and therefore takes much longer to reach this limit than someone who goes straight into a hot environment. But they still stop at around the same temperature, according to this lab research.

Remember that this is found when humans exercise in a laboratory at a constant workload until they themselves decided “enough is enough” and choose to stop. When given a little more motivation (like when an Olympic gold medal is on the line, or that 10km PB you’ve been training for), it’s likely that you’ll get this body temperature up to 41 degrees, but beyond that, it seems that exercise is very nearly impossible, at least in the absence of some pathology or abnormal response.

Remember also that heat stroke, which is a very serious medical condition, happens at a temperature of 42 degrees, so the limit to voluntary exercise happens well before this level is attained. That of course raises the interesting question of why heatstroke happens – a malfunction of the “off-switch”, perhaps? Or a failure of the signal to actually reach the brain to stop exercise? It’s a difficult one, for which there are theories, and we’ll cover them at some stage.

The mechanism – reduced muscle activation and arousal levels

So once this was discovered, science began looking for the mechanism, the HOW of the “off-switch”. Because the thinking was that the central drive (from the brain) was the culprit, it made sense to look at brain function for clues, and that’s exactly what the Danish researchers did. So, in a series of studies, cyclists were made to ride in the heat at a fixed workload until exhaustion, and then various measurements were made of brain function and muscle function. There were two key findings:

At very high (40 degrees) body temperatures, immediately after the athletes had become exhausted, they found that the activation of muscle by the brain was actually LOWER than when the body temperature was only 38 degrees. The graph below shows the EMG activity in the quadriceps muscles after exercise in the hot and cool conditions. It’s quite clear that the EMG, which is a measure of activation of muscle, is lower when the body is hot. So that gives an indication of why the cyclists were no longer able to push out the required force – their brain simply prevented them from activating the required amount of muscle.

There was evidence of reduced arousal/motivation levels once the body temperature rose. In fact, what was found is that there was a very good correlation between a rise in body temperature and a reduction in arousal. Motivation or arousal, incidentally, was measured using EEG and the ratio of certain brain waves which are known to indicate this parameter. The key point here is that as the body temperature gets higher, the motivation declines, and this in turn is responsible for a rise in the perception of effort. They therefore found a good correlation between RPE and a rise in body temperature, though of course, correlations are often a slightly misleading. The key is: Increased body temperature = decreased motivation/arousal = increased effort perception.

The problem with this research: What happens before the “off-switch” is reached?

Again, the key question one should be asking is whether this solid science is actually relevant to what you are going to witness in Beijing later this year? Because in Beijing, the world’s best atheltes will line up, highly motivated, take part in a race, where they can speed up or slow down, depending on the innumerable factors that go into racing strategy.

The studies have shown that when athletes go at a fixed pace until they are exhausted, they’ll stop when their body temperature hits about 40 degrees. Perhaps, given the incentive of Olympic Gold, that temperature will be higher. And perhaps, when they push themselves hard enough for the rewards that are on offer in Beijing, they’ll be able to raise their body temperatures so high that they end up looking like the famous Gabrielle Andersen from the 1984 Olympic Games marathon (see video below).

But, realistically, you know that this doesn’t happen, because Olympic competition is not a fixed workload trial to exhaustion in a lab, and the athlete is able to slow down if they wish. And so what you will see in Beijing is athletes dropping off the lead pace after only 7 km of a 10km race, and then you have to wonder: Are they hot, or is something else in play? And you should be asking: What happens when the body temperature is 39 degrees, and there are still 20 km of the marathon left to run? Does the brain allow the athlete to just run and run until it the body temperature hits 40 degrees, and the athlete stops? Of course, you suspect the answer is no.

So the “limitations” theory for exercise in the heat, while proven in the lab, fails to explain what you’ll see in Beijing later this year, and will have experienced in your own training, many times.

Did you know?

We published The Runner's Body in May 2009. With an average 4.4/5 stars on Amazon.com, it has been receiving positive reviews from runners and non-runners alike. Available for the Kindle and also in paperback.